A driving system for a work machine, such as a hydraulic excavator, may include a hybrid drive system that has an electric generator, which is adapted to be driven by an engine, and an electric power storage device for storing electric power generated by the generator. A hydraulic actuator control circuit for a boom cylinder, a stick cylinder, and a bucket cylinder, etc. includes an electric motor or a motor generator, as well as a pump or a pump motor to be driven by the electric motor or the motor generator. The aforementioned electric motor or motor generator is operated by power supplied from either one of or both the generator and the electric power storage device of the hybrid drive system. For example, a boom cylinder control circuit may include a bi-directional type pump motor and a motor generator. The bi-directional type pump motor is adapted to function as a pump for feeding hydraulic fluid and also function as a hydraulic motor driven by hydraulic fluid fed thereto. The motor generator is adapted to be driven by electric power supplied from the generator or the electric power storage device so as to function as an electric motor for driving the pump motor and also adapted to be driven by the pump motor so as to function as a generator for generating electric power (e.g. Japanese Laid-open Patent Publication No. 2004-190845 (page 7, and FIG. 1)).